Cathode ray tubes (CRTs) used in many electrical measurement instruments have a graticule imprinted on their faceplates for measuring the waveform produced by the beam in response to an electrical signal. The deflection of the electron beam must be periodically calibrated to the graticule to ensure the deflection is accurate. Until the present invention, this calibration has largely been done manually. A highly accurate input signal is applied to the amplifying means driving the CRT to deflect the electron beam to a graticule line that correctly measures the value of the input signal. A human operator then visually determines the position of the beam relative to the graticule line and manually adjusts the amplifying means until the beam coincides with the line.
This manual approach is expensive because it is slow and labor intensive. It is also relatively inaccurate. The electron beam is wider than the graticule lines, making it difficult to visually align the center of the beam with the center of the graticule line.
Despite these drawbacks, this approach has remained the prevalent method of calibration, although techniques for locating the position of the beam automatically have been known for some time. For example, beam indexing cathode ray tubes employed for color television applications can accurately locate the tube's electron beam so as to register properly with the separate color-emitting phosphors of the primary colors. U.S. Pat. Nos. 4,257,869; 2,778,971; and 2,790,107, and British Pat. No. 822,017 illustrate cathode ray tubes that employ indexing elements, strips, or bands supplying feedback indicative of an electron beam's actual position. The indexing elements produce a light emission or electron emission when crossed by an electron beam and this emission is detected within the tube in order to energize the beam at the correct time. Such tubes, however, require sophisticated and complex screen techniques to manufacture and therefore are expensive to produce.
Other methods that have been tried include employing an electronic graticule or a television camera to record the beam striking the CRT faceplate. These methods, too, are relatively slow and expensive.